In general, an inverter as the alternating current power supply device generates a voltage of several hundreds to one thousand and hundreds of volts at a frequency of several dozens Hz and applies the voltage on a discharge lamp, for example, CCFL (Cold Cathode Fluorescent Lamp) to light on. The discharge lamp and the inverter are therefore used in combination.
The inverter comprises a direct current power supply, a bridge circuit consisting of switch elements by which a direct current electric power of the direct current power supply is converted into an alternating current power, a pressure transformer that pressurizes a voltage of the alternating current power converted in the bridge circuit to light on the discharge lamp, a current detecting circuit for detecting a current flowing through the discharge lamp, a control circuit that turns on/off the switch elements so that the current may reach a prescribed value based on the current detected by the current detecting circuit and its feedback loop.
Note here that there are two cases: one is that the pressure transformer is provided, between a primary winding and a secondary winding, with an insulation function; and another is that the pressure transformer is provided, therebetween, with no insulation function. The former case is called to as “insulation type system”, while the latter is called to as “no-insulation type system”.
In the non-insulation type system, the whole system is operated as the secondary side. In general, the inverter utilizes, as an input power supply thereof, a voltage of a DC/DC converter in the preceding stage of the inverter. This voltage is a secondary side voltage because it has already been insulated by the DC/DC converter. Moreover, since the voltage that has already been controlled by the DC/DC converter is input, the input voltage has a substantially constant value when viewing from the inverter, and it is not necessary to consider a wide range of variable input. In addition, as the pressure transformer is not required to have the insulation function, there is little restriction on the safety standard, allowing the non-insulation type system to be realized small and at a low price.
However, as the DC/DC converter is indispensable, the above non-insulation type system has two steps of electric power conversion stages (i.e. the DC/DC converter and the inverter). Thus, this non-insulation type system is disadvantageous in view of its efficiency and also in the price of the DC/DC converter.
On the contrary, the insulation type system can use voltage obtained by rectifying the alternating voltage as it is, as the power supply to be inputted to the inverter. Thus, owing to one step of electric power conversion stage, the insulation type system is advantageous in view of its efficiency and also advantageous in the price since the DC/DC converter can be deleted. However, as the input voltage of the inverter is nothing but the rectified alternating voltage, the variable range of voltage is large. It is therefore difficult to control the output current flowing through the discharge lamp to a constant value.
Moreover, in particular, the impedance of a discharge lamp a cold cathode fluorescent lamp has a negative resistance characteristic in general. In addition, as the brightness characteristic of a cold cathode fluorescent lamp is greatly ruled by the current flowing through the cold cathode fluorescent lamp, it is general to control the current value flowing through the cold cathode fluorescent lamp. For instance, the current value of a cold cathode fluorescent lamp is controlled since the electric power to be supplied to a transformer is changed by controlling the frequency of a switch element in a bridged circuit or controlling the duty ratio (simply referred to as “duty” after) of ON/OFF at the switch element.
However, in recent liquid crystal TVs etc., it is often the case that an interference of a driving frequency of the inverter with the clock frequency of a controller in a TV device causes a problem. It is therefore required to perform the control operation having a fixed frequency causing no problem. In this case, PWM control is used as a method of controlling the switch element of the bridged circuit. In the PWM control, the output electric power is controlled by turning ON/OFF one switch element for high side and another switch element for low side alternately, the switch elements being connected in series and also connected to both ends of the direct current power supply, while changing their ON-widths (i.e. changing their ON-duty). The greater the ON-widths do get, the larger the output power gets.
On the other hand, there is a case that the value of the direct current voltage identical to the input voltage of the inverter does change. In the notebook computer etc., for instance, there is a great change of 8V to 20V in the input voltage since the computer may be driven by batteries or may be driven through an adapter. Moreover, in case of a system directly using voltage obtained by rectifying the alternating current voltage, such as liquid crystal TV and liquid crystal monitor, there is a possibility of great change in voltage. In case of a device for a wide range of alternating current voltages, it would be subjected to a greater change in voltage.
Thus, in spite of variance in input voltage, the PWM control for the bridge circuit allows the current in the discharge lamp to be ideally controlled to a constant value since the duty is increased when the input voltage is small and alternatively, the duty is reduced when the input voltage is large. However, the change might take place in the current of the discharge lamp due to a change in input voltage. As the cause, there may be expected the following points. First, the gain of a feedback loop is small. Secondarily, the current waveform of the discharge lamp to be detected changes to cause a variation of the detected value changes consequently. In this way, the brightness of the discharge lamp is changed due to the change in the current in the discharge lamp.
Japanese Patent Publication Laid-open No. 6-68979 discloses a discharge lamp lighting device which is adapted so as to maintain the lighting of a discharge lamp at usually-stable brightness even if the voltage supplied to a lighting circuit changes by the change of the input voltage etc. This discharge lamp lighting device is adapted so as to detect a current value flowing in a switching circuit. The discharge lamp lighting device further includes a comparator that compares the detected current value with a current value defining the dimming value of the discharge lamp and outputs voltage corresponding to a current difference between the former current value and the latter current value, an oscillating circuit that changes its oscillation frequency corresponding to the voltage from the comparator and a control circuit for turning ON/OFF the switch element corresponding to the oscillation frequency from the oscillating circuit. With the above constitution, the disclosed discharge lamp lighting device is adapted so as to supply an electric load with prescribed current. In this device, it is necessary to enlarge the gain of the above-constructed feedback loop (which is equivalent to the gain of the comparator) to prevent the current flowing through the electric load from being varied due to the change of the power supply voltage.